European patent EP 1 333 304 B1 discloses an autofocus module and an autofocus method for a microscope-based system. To this end at least two light sources are provided, each of which produces a light beam for focusing. An optical means is provided, which directs a fraction of each light beam onto a coupling means coupling each light beam into the illumination light beam of the microscope-based system and directing it onto the object. A first and a second detector receive the light beam of the first and the second light source reflected from the surface of the object to be studied, and determine the intensities on the first and on the second detector by time division multiplexing.
U.S. Pat. No. 6,812,446 B2 describes an autofocus module for a system based on a microscope, in which a light source for a measurement light beam is provided. A first axicon generates an excentric, annularly diverging measurement light beam. A second axicon is provided for parallelising the remitted divergent measurement light beam. A differential diode is provided for determining the focus position.
U.S. Pat. No. 6,875,972 B2 relates to an autofocus module exhibiting a cylindrical lens between a detector element and a further optical means. For determining the focus position the cylindrical lens generates a line on the detector element. The detector element is rotatable about an axis in such a way that it is tilted to a settable degree with respect to a plane defined by the surface of the sample. All optical components of the autofocus module are arranged in a housing, which can easily be connected to an existing microscope-based system.
German patent application DE 10 2005 022 125 A1 describes a light-based scanning microscope with an excitation light path and a detection light path, means for scanning an object by moving an imaged spot, line, or multispot range over the object and an objective for imaging the spot, line, or multispot range. A focus setting mechanism is provided for the objective; the focus plane of the objective is detected by an autofocus system, which images different ranges of depth at the imaged spot, line, or multispot range to different locations of a space-resolving detector.